Wide dynamic range omnidirectional optical sensor for detecting nuclear detonations

ABSTRACT

An optical sensor including a photosensitive device and a linear network and a logging network connected in series with the photosensitive device and with a potential source so that the photosensitive device will operate in the voltaic mode under conditions of low light and will operate in the photo current mode under conditions of high ambient light to produce a wide dynamic range of sensing of light flashes. The sensor units are particularly adaptable to the ganging of plural sensors with amplifiers to permit the arrangement of the photosensitive devices of the sensors for omnidirectional operation.

United States Patent 1191 Weischedel Jan. 7,1975

[54] WIDE DYNAMIC RANGE 3,147,380 9 ll964 Buckingham, et a1. .1 250/338 OMNIDIRECTIONAL OPTICAL SENSOR 3,428,814 2/1969 Doonan 250/214 FOR DETECTING NUCLEAR Primary Examiner-Archie R. Borchelt DETONATIQNS Assistant Examiner-T. N. Grigsby [75] Inventor: Richard C. Welschedel, Camillus, Attorney Agent, Firm Car| w Baker; Frank L Neuhauser; Richard V. Lang [73] Assignee: General Electric Company, New

York, NY. [57] ABSTRACT [22] Filed: AP 3, 1973 An optical sensor including a photosensitive device and a linear network and a logging network connected PP 347,611 in series with the photosensitive device and with a potential source so that the photosensitive device will 52 US. Cl 250/206, 250/336, 250/338, operate in the voltaic mode under conditions of low 250/349 light and will operate in the photo current mode under 51 Int. Cl G0lj 1/16, GOlt 1/16 conditions of high ambient light to Produce a Wide [58] Field of Search 250/206, 555, 556, 336, amic range of ensing of light flashes, The sensor 250/338 349 units are particularly adaptable to the ganging of plural sensors with amplifiers to permit the arrangement [56] References Cited of the photosensitive devices of the sensors for omni- UNITED STATES PATENTS drecficnal 2,804,574 8/1957 Kingsbury 250/206 x 1 Claim, 2 Drawing Figures LINEAR NET LOGARITHMIC NET Patented Jan. 7, 1975 3,859,519

1 LINEAR NET LOGARITHMIC NET w SENSORA BUFFER AMPLIFIER A BUFFER AMPLIFIER B SENSORB WIDE DYNAMIC RANGEOMNIDIRECTIONAL OPTICAL SENSOR FOR DETECTING NUCLEAR DETONATIONS 2 g the photo current mode in series with an electronic circuit comprising logarithmic and lirfear networks. The

potential across the logarithmic and linear networks will be the sumi of ihe potential across each. Trhe output "tl'i e event itself by measurement of time between the 5 of each sensing unit is ac. coupled to an operational BACKGROUND. OF THE INVENTION half-wave buffer amp ljfieri The outputs and feedback 1. Field of the Invention m p harh's of the buffer' 'a'mplifier are connected so that only The invention relates to .t lre artiof-tthe" detection of the higher peak output is obtained. With the photosennuclear events or ot h egligfi t flashes. In the case of nu- ,sitive-defvices connected in series with both a linear reclear y entsait*is'possible to measure the intensity of sistance and circuitry producing a logarithmic function, the sensing unit has a sensitivity inversely proportional ter intensity.

occurrence of the first thermal and the second thermal flashes. For this purpose devices are needed which are capable of detecting and reporting the flashes caused to the ambient light at low light ambient levels and has .a fixed sensitivity at high ambient light levels. The output of the sensingunit is then subjected to processing by the thermals. These devices can be either directional to limit the ultimate output to only the'highest amplior omnidirectional. For the purposes of around the clock area monitoring they should also be capableof operating under all light conditions.

2. Prior Art Optical sensors have-been made for many purposes.

Some have been designed and used for detecting nuclear events and normally employ silicon voltaic cells which are operated in either the ivoltaic mode or the photo, current mode. In the voltaic mode the output of the cell is a function of the logarithm of the current in tensity which provides a fairly wide dynamic range except for the fact that the sensitivity is poor under high ambient light conditions. In thevoltaic mode of operation the photosensitive device can operate at a threshold level on the order of magnitude of the ambient con 3 ditions which causes the device in the voltaic mode to be fairly sensitive under conditions of 'lowliambient light. However, under high ambient light conditions, as

for example bright sun, such devices are insensitive to,

light which might be 100 milliwatts per square centime- If the photosensitive device is opeliatediin thejpihoto current or leakage mode ofoperation it produces an output which is linear with light intensity." This provides good sensitivity under conditions of high ambientlight, K but results in poor sensitivity in low-flight levelsland lj' therefore'has a limited dynamic range,

The prior, artdoes not provide 7 for the detection and recording of'nuclear events which will work in the sunlight which might have; an intensity? on the order of mwlcm as wellasworking under low light conditions without producing false alarmsn potential saturation problems. 7 SUMMARY OF THE INVENTION The principal object of this invention isto provide an optical sensor which will operate over a .wide range of ambient light conditions so that it may be used as a sensor of for example the light generated by a nuclear event under the full range of IightcOndition'sfrornbright sunlight to dark. This result can be accomplished by joining the desirable characteristics of the voltaic mode (logarithmic)and photo current mode (linear) of operation of a photosensitive device. One possible im-f plementatiorl is by providing a variable load resistance tude'signal. I

. DESCRIPTION OF THE DRAWING rectional optical-sensor. I

DESCRIPTION OF THE PREFERRED EMBODIMENT The embodiment of the invention as shown in schematic diagram inFIG. 1 comprises a single sensing unit *1 including a terminal 2' for application of a potential, photosensitive device 3,.a linear network 4, a logarithmic network 5, and'a; ground 6, all connected in series. When a potential such as the B+ indicated is appliedto the circuit, thephotosensitive device 3 is operated in the reverse bias leakage condition,-i.e., the photo currentmode, wherein thephoto current I produced is directlyproportional totheiincident light intensity. The photo current from the device is applied to the linear andllogarithmic networks inseries as shown in FIG. I.

- An output takenatterminal7-isthe sum of the individjual potential drops across the two networksfThe linear network4 is principally resistive, and will-cause a poten tial drop over the net iof V IR. .The logarithmic ;network is designed so; that the potential drop across j j the ne'tis-acc'ording to V- 'log" I,.With the photosensifor an' optical sensor 'tive device 3 conducting and -currem flowing in the sensingunitcir cuit; the potential at any time at termiresents-the characteristics of the'electronic component's'used-in the logging netglnFlG. 2 this same sensing unitfcircuit arrange mentis' "used'as both sensor A and sensor B in which both linear network and the logarithmic network have been reduced to their most sim- 'ple forms of a resistor 40 and asilicon diode 50, al-

fthough any appropriate electronic'components producing'the same result could be used.

With more specific reference to FIG. 2, the sensing unit can be used in combination'with a device such as the buffer amplifier A to limit the-output to that produced by'changes in lightincident on the photosensitive device which deviate frorn-"ithe ambient light level. a As also shown in FIG. 2 multiple sensors can be combined as desired to build an omnidirectional device in spite of the fact that the individualphotosensitive devicesused'mayhave quite directional characteristics. In the implementation of FIG. 2, sensors'A and B are "identical andamplifiers A and B are identical and are connected to a common output terminal 36. Each buffer amplifier'which is ac. connected to the sensing unit by means of capacitor 9, 91 includes a grounded resistor 22, 23, an operational amplifier 24, 25 and a feedback circuit including resistances 28, 29 and diodes 26, 27 which with resistors 32, 33 and 34, 35 provide a nonlinear network. Since the operational amplifier 24 is connected between capacitor 9 and resistor 22 which provide for a decay time longer than the signal of interest which is on the order of 10-20 seconds, the amplifier sees only changes occurring in the photo current in the sensing unit. This method of connecting the amplifier to the sensing unit removes from further consideration the photo current resulting from the ambient light level and therefore causes the output of the amplifier as delivered to terminal 36 to be dependent upon optical events or other light deviations rather than on steady state conditions. Diodes 38, 39 are used to permit the coupling of a plurality of sensing unit and buffer combinations to a common output such as terminal 36.

Use of these diodes 38, 39 which provide a low impedance for positive loads (forward bias) and high impedance for negative loads (reverse bias) permit only the higher amplitude signals with their positive loads to be transmitted to the output terminal.

Not only does this arrangement solvethe problem of 10 at capacitor 9 in FIG. 2) with each incremental increase of 10 mwlcm Therefore for increases in light of less quantity than 10 mw/cm the voltage across re sistance 40 will be low (i.e., less than 45 mv). For 100 percent light increases below that same level of l l mw/cm the voltage across logging network will be 45 mv. For example, as light would increase from 0.l mw/cm ambient to 0.2 mw/cm, the voltage change AV across resistance 40 and the diode forward voltage drop change Al/F would be 0.5 and 45 mv respec tively. The following table shows this over the dynamic range of 0.1 mw (night time) to over 200 mw/cm (twice full sun intensity). Therefore the following table shows that the sensitivity varies inversely with ambient light at low light levels and is fixed at high light levels:

AMBIENT VALID* OUTPUTS (mv) LIGHT SIGNAL VOLTAIC PHOTO- SENSOR 1 COMMENTS mw/cm INTENSITY MODE CURRENT V. V,

REQUIRED MODE mw/cm R 450 .1 .l 45 0.45 45.45 Voltaic V .2 .2 45 0.90 45.90 Sensitivity .4 .4 45 L80 46.80 Range varies .8 .8 45 3.60 48.60 inversely with .9 4] 4.05 45.05 ambient light. 2 1.7 40 7.65 47.65 4 3 I35 48.5 Both voltaic 8 5 25 22.5 47.5 and photo- IO 5.5 21 24.7 45.7 current 20 7 l8 31.5 49.5 sensitivity. 8 lo 36 46 Photocurren! 80 I0 5 Range Fixed I00 10 5 45 45+ Sensitivity I50 10 5 45 45+ 200 l0 5 45 45+ Valid signal is defined as one which produces 45 lo 50 millivolt output.

attaining the desirable sensitivity in any level of background light but it provides a system in which the sensitivities can be controlled. The voltage drop across the silicon diode 50 or similar logging network 5 is basically the log of the current (V= log I) and increases approximately 45 millivolts for each percent increase in diode current. Of course this is significant only in establishing the sensitivity under low light level conditions because of the a.c. coupling of the sensor to the buffer amplifier. The sensitivity itself of the logging network can be controlled so as to avoid false alarms by light flashes less than that desired to be reported by maintaining a d.c. bias through the diode. A d.c. bias through the diode may be maintained at any threshold level by means of an arrangement such as the resistance 8 in parallel with the photosensitive device 3 to give an equivalent of a minimum light level sensing. The resistance for the linear network 4 or resistor 40 is chosen so that at low light levels the voltage across it is insignificant, i.e. much less than 45 millivolts or some other desired level which is coordinated with the value of resistor 8.

As already indicated the operational amplifier 24 has its noninverting input attached between the capacitor and the grounded resistor 22 and sees only the varying d.c. potential appearing the the sensing circuit. Any change in level of potential applied to capacitor 9 is amplified by the non-inverting amplifier 24 to produce a potential at the input side of diode 38 which is equal to the input voltage times the sum of the resistances of diode 38, resistance 34 and resistance 32 which sum is divided by resistance 32. Noting that the voltage at the input side of diode 38 could also be expressed as equal to the output voltage at the output of the buffer amplifier times the sum of the resistances of the diode 38, resistance 34 and resistance 32, said sum being divided by the sum of resistors 34 and 32, it would appear that the output voltage of the entire system in terms of the input voltage to operational amplifier 24 would be equal to that input voltage times the sum of the resistances of resistors 34 and 32 which quantity is divided by the resistance of resistor 32. This may be algebraically expressed as follows:

V13 12 es 34 a2I/ a2;

V13 V14 (R33 R32 R34)/R34 R32; and, therefore:

Where R represents the resistances of a component having the subscript number and wherein the V represents the voltage at the point on FIG. 2 at the subscript number. This demonstrates that the voltage across diode 38 drops out of the gain equation making possible the use of the diodes 38, 39 solely for the purpose of ganging sensing units and buffer amplifiers into an integrated net to produce an omnidirectional system. As previously noted, diodes 38, 39 permit the output at terminal 36 to reflect only the highest amplitude received from the individual sensors. This is demonstrated by noting that if the signal from photosensitive device 3 is higher than that from device 31 the amplifier A will have a higher output at 13 than buffer amplifier B will produce at 16. Under these conditions amplifier 25 output will go negative at 16 to hold its noninverting input equal to the voltage at 17 and the diode 27 will provide a low impedance path for this signal causing a voltage to develop across resistance 35 equal to the ratio of the difference of the photosensitive device outputs. Resistor 35 and diode 29 are both high impedance compared to diode 38 under these conditions since diode 38 is forward biased and diode 39 is back biased. Any number of stages can be used providing the positive output impedance of any amplifier is much lower than the parallel negative load impedances of the remaining amplifiers. Since the resistances of re- 1. An optical sensor for detecting and signaling the occurrence of a deviation from an ambient light condition comprising: I

a. an electrical photosensitive device;

b. electrical terminal and ground means for applying a potential across said photosensitive device;

c. a variable load resistance means in series with said photosensitive device for producing an output sig nal in terms of the potential across said resistance means which is relatively linear for high current flows and relatively logarithmic for low current flows whereby said device and said resistance means constitutes a detector circuit, said variable load resistance means including:

1. a linear network of resistance R across which the potential V is according to V= IR, in series with 2. a logarithmic network across which the potential Vis according to V= (I) log I, whereby said variable load means output signal is a voltage according to V= (f) log I IR; and

d. a resistance in parallel with said photosensitive device for providing a threshold level of current in said detector circuit. 

1. An optical sensor for deteCting and signaling the occurrence of a deviation from an ambient light condition comprising: a. an electrical photosensitive device; b. electrical terminal and ground means for applying a potential across said photosensitive device; c. a variable load resistance means in series with said photosensitive device for producing an output signal in terms of the potential across said resistance means which is relatively linear for high current flows and relatively logarithmic for low current flows whereby said device and said resistance means constitutes a detector circuit, said variable load resistance means including:
 1. a linear network of resistance R across which the potential V is according to V IR, in series with
 2. a logarithmic network across which the potential V is according to V (f) log I, whereby said variable load means output signal is a voltage according to V (f) log I + IR; and d. a resistance in parallel with said photosensitive device for providing a threshold level of current in said detector circuit.
 2. a logarithmic network across which the potential V is according to V (f) log I, whereby said variable load means output signal is a voltage according to V (f) log I + IR; and d. a resistance in parallel with said photosensitive device for providing a threshold level of current in said detector circuit. 